Studies of the biochemical processes by which yeasts metabolize alkanes and fatty acids have revealed three types of oxidation reactions: α-oxidation of alkanes to alcohols, ω-oxidation of fatty acids to α, ω-dicarboxylic acids and the degradative β-oxidation of fatty acids to CO2 and water. The first two types of oxidations are catalyzed by microsomal enzymes while the last type takes place in the peroxisomes. In C. tropicalis, the first step in the ω-oxidation pathway is catalyzed by a membrane-bound enzyme complex (ω-hydroxylase complex) including a cytochrome P450 monooxygenase and a NADPH cytochrome reductase. This hydroxylase complex is responsible for the primary oxidation of the terminal methyl group in alkanes and fatty acids as described, e.g., in Gilewicz et al., Can. J. Microbiol. 25:201 (1979), incorporated herein by reference. The genes which encode the cytochrome P450 and NADPH reductase components of the complex have previously been identified as P450ALK and P450RED respectively, and have also been cloned and sequenced as described, e.g., in Sanglard et al., Gene 76:121-136 (1989), incorporated herein by reference. P450ALK has also been designated P450ALK1. More recently, ALK genes have been designated by the symbol CYP and RED genes have been designated by the symbol CPR. See, e.g., Nelson, Pharmacogenetics 6(1):1-42 (1996), which is incorporated herein by reference. See also Ohkuma et al., DNA and Cell Biology 14:163-173 (1995), Seghezzi et al., DNA and Cell Biology, 11:767-780 (1992) and Kargel et al., Yeast 12:333-348 (1996), each incorporated herein by reference. In addition, CPR genes are now also referred to as NCP genes. See, e.g., De Backer et al., Antimicrobial Agents and Chemotherapy, 45:1660 (2001). For example, P450ALK is also designated CYP52 according to the nomenclature of Nelson, supra. Fatty acids are ultimately formed from alkanes after two additional oxidation steps, catalyzed by alcohol oxidase as described, e.g., in Kemp et al., Appl. Microbiol. and Biotechnol. 28: 370-374 (1988), incorporated herein by reference, and aldehyde dehydrogenase. The fatty acids can be further oxidized through the same or similar pathway to the corresponding dicarboxylic acid. The ω-oxidation of fatty acids proceeds via the ω-hydroxy fatty acid and its aldehyde derivative, to the corresponding dicarboxylic acid without the requirement for CoA activation. However, both fatty acids and dicarboxylic acids can be degraded, after activation to the corresponding acyl-CoA ester through the β-oxidation pathway in the peroxisomes, leading to chain shortening. In mammalian systems, both fatty acid and dicarboxylic acid products of ω-oxidation are activated to their CoA-esters at equal rates and are substrates for both mitochondrial and peroxisomal β-oxidation (J. Biochem., 102:225-234 (1987)). In yeast, β-oxidation takes place solely in the peroxisomes (Agr. Biol. Chem. 49:1821-1828 (1985)).
Cytochrome P450 monooxygenases (P450s) are terminal monooxidases of a multicomponent enzyme system including P450 and CPR (NCP). In some instances, a second electron carrier, cytochrome b5(CYTb5) and its associated reductase are involved as described below and in Morgan, et al., Drug Metab. Disp. 12:358-364 (1984). The P450s comprise a superfamily of proteins which exist widely in nature having been isolated from a variety of organisms as described e.g., in Nelson, supra. These organisms include various mammals, fish, invertebrates, plants, mollusk, crustaceans, lower eukaryotes and bacteria (Nelson, supra). First discovered in rodent liver microsomes as a carbon-monoxide binding pigment as described, e.g., in Garfinkel, Arch. Biochem. Biophys. 77:493-509 (1958), which is incorporated herein by reference, P450s were later named based on their absorption at 450 nm in a reduced-CO coupled difference spectrum as described, e.g., in Omura et al., J. Biol. Chem. 239:2370-2378 (1964), which is incorporated herein by reference.
Monooxygenation reactions catalyzed by cytochromes P450 in a eukaryotic membrane-bound system require the transfer of electrons from NADPH to P450 via NADPH-cytochrome P450 reductase (CPR) as described, e.g., in Taniguchi et al., Arch. Biochem. Biophys. 232:585 (1984), incorporated herein by reference. CPR is a flavoprotein of approximately 78,000 Da containing 1 mol of flavin adenine dinucleotide (FAD) and 1 mol of flavin mononucleotide (FMN) per more of enzyme as described, e.g., in Potter et al., J. Biol. Chem. 258:6906 (1983), incorporated herein by reference. The FAD moiety of CPR is the site of electron entry into the enzyme, whereas FMN is the electron-donating site to P450 as described, e.g., in Vermilion et al., J. Biol. Chem. 253:8812 (1978), incorporated herein by reference. The overall reaction is as follows:H++RH+NADPH+O2→ROH+NADP++H2O
Binding of a substrate to the catalytic site of P450 apparently results in a conformational change initiating electron transfer from CPR to P450. Subsequent to the transfer of the first electron, O2 binds to the Fe2+-P450 substrate complex to form Fe3+-P450-substrate complex. This complex is then reduced by a second electron from CPR, or, in some cases, NADH via a second electron carrier, cytochrome b5 (CYTb5) and its associated NADH-cytochrome b5 reductase as described, e.g., in Guengerich et al., Arch. Biochem. Biophys. 205:365 (1980), incorporated herein by reference, and Morgan, supra. Most of the aforementioned studies implicate CYTb5 as being involved in the pathway only for the transfer of the second electron. One atom of this reactive oxygen is introduced into the substrate, while the other is reduced to water. The oxygenated substrate then dissociates, regenerating the oxidized form of the cytochrome P450 as described, e.g., in Klassen, Amdur and Doull, Casarett and Doull's Toxicology, Macmillan, New York (1986), incorporated herein by reference. With respect to the CYTb5, several other models of the role of this protein in P450 expression have been proposed besides its role as an electron carrier.
While several chemical routes to the synthesis of long-chain α,ω.-dicarboxylic acids as 9-octadecenedioic acid are available, such methods are complex and usually result in mixtures containing shorter chain lengths. As a result, extensive purification steps are necessary. As an alternative to chemical syntheses, long chain α,ω-dicarboxylic acids such as 9-octadecenedioic acid can be made via fermentation methods such as microbial transformation of the corresponding hydrocarbons such as alkanes or alkenes, fatty acids or esters thereof. One method for producing substantially pure α,ω-dicarboxylic acids in substantially quantitative yield is described in U.S. Pat. No. 5,254,466, the entire contents of which are incorporated herein by reference. This method comprises culturing a C. tropicalis strain wherein both copies of the chromosomal POX5 and each of the POX4A and POX4B genes are disrupted in a culture medium containing a nitrogen source, an organic substrate and a cosubstrate.
The POX4 and POX5 gene disruptions effectively block the β-oxidation pathway at its first reaction (which is catalyzed by acyl-CoA oxidase) in a C. tropicalis host strain. The POX4A and POX5 genes encode distinct subunits of long chain acyl-CoA oxidase, which are the peroxisomal polypeptides (PXPs) designated PXP-4 and PXP-5, respectively. The disruption of one or more of these genes results in a partial or complete inactivation of the β-oxidation pathway thus allowing enhanced yields of dicarboxylic acid by redirecting the substrate toward the α-oxidation pathway and also prevents reutilization of the dicarboxylic acid products through the β-oxidation pathway.
Another method for producing substantially pure α,ω-dicarboxylic acids in substantial yield is described in U.S. Pat. No. 6,331,420, the entire contents of which is incorporated herein by reference. This method includes increasing the CYP and CPR (NCP) enzymes by amplification of the CYP and CPR gene copy number in a C. tropicalis strain, and culturing the genetically modified strain in media containing an organic substrate.
Gene(s) involved in the bioconversion of various feed stocks, e.g., HOSFFA (high oleic sunflower oil, i.e., fatty acid mixtures containing oleic acid commercially available from Cognis Corp. as Edenor® and Emersol®), have native promoters that control their transcriptional regulation. These promoters are sometimes inadequate to achieve the level of transcription needed to make a gene(s) product, that is involved in a given process. Accordingly, there exists a need for tailored promoters which can aid in pocesses for increasing dicarboxylic acid production in yeast.